OS detection in config.mk without using uname

GCC has an option -dumpmachine which provides the necessary info.
Use just-built dynamic RTS during library typecheck

Idris has been extended with a flag to choose which dynamic RTS library will
be loaded on startup. The build system has been updated to build the dynamic
RTS after Idris but before typechecking the stdlib. Typechecking the stdlib
now uses the just-build dynmic RTS in dist/.
Added new tests to the test README